Hurricane Sandy is now moving through the NW Bahamas, and has weakened since yesterday. The center is now exposed due to southwesterly wind shear imposed by the subtropical jet. This weakening was expected, but has occurred a bit sooner than anticipated. This is good news for the northern Bahamas and eastern Florida, which are not receiving as big of a lashing as they would have. Despite having an exposed center, Sandy remains a solid hurricane, and her central pressure is still below 980mb. As Sandy’s center passes through the subtropical jet core, some additional weakening may occur, but she will likely remain a hurricane. As Sandy begins to accelerate northeastward in 24 hours or so, upper winds over the storm will lighten a bit, and upper-level dynamics will continue to improve, with significant divergence in the 200mb wind field expected to develop in the path of the storm to the north and northwest. Thus, some restrengthening is expected in the maximum winds. The increase in wind will likely remain limited due to the ever-expanding nature of Sandy’s wind field, but her central pressure is expected to drop significantly due to baroclinic dynamics as she nears the Atlantic coast. As a result, an area of tropical storm force winds several hundred miles in diameter will affect the northeastern U.S., and hurricane force winds will likely affect the coastline where the storm moves ashore.

The track forecast philosophy for Sandy remains unchanged. An acceleration to the NE or NNE as the ridge to the north moves out of the way will occur during the next day or two, bringing Sandy up between Cape Hatteras and Bermuda. The storm’s structure has become weighted to the north and northwest as expected, and this will allow the storm to interact positively with a mid-latitude trough moving into the eastern United States. The resulting phasing will draw the storm towards the north and northwest, with a sharp left turn likely before an eventual landfall on the mid-Atlantic or New England coast. The forecast track has been shifted a bit southward Days 4-5, and remains in good agreement with the consistent ECMWF ensemble mean, but lies still north and east of the ECMWF operational.

Sandy has the potential to become a historic storm for the northeast U.S., and residents should be prepared for hurricane conditions along the coastline and tropical storm conditions well inland and well away from where the center of circulation comes ashore. Due to baroclinic enhancement, Sandy’s circulation continues to expand, and tropical storm force winds will extend over 300 miles from the center at landfall. Prolonged heavy rain and storm surge are likely to cause severe damage in some areas, and residents are urged to treat this as another Irene-type event.

Hurricane Sandy strengthened at a somewhat unprecedented rate last night, and made landfall in eastern Cuba as a strong Cat 2. Sandy is now back over the water and is lashing the Bahamas as a slightly weaker Cat 2 hurricane. Some restrengthening is likely as the storm moves through the Bahamas, and Sandy could potentially become a Cat 3 hurricane, though due to dry air likely to start plagueing the storm permanently now that the core has been disrupted, the forecast keeps her peak at upper-end Cat 2 intensity. Regardless, the Bahamas will be lashed for at least the next 24-36 hours due to a slowing of Sandy’s forward movement in response to a steering flow change. Thereafter, as Sandy moves away from the Bahamas, her maximum winds are likely to slow, despite the fact that the central pressure will likely remain steady or fall. This will be due to an expansion of Sandy’s windfield, such that the core will not be as tight as it is now. The intrusion of dry air into Sandy’s massive circulation will be partially responsible for this, along with baroclinic influence from a mid-latitude trough to the northwest. Sandy will likely retain hurricane-force winds through her probable landfall somewhere in New England as she begins to take on extratropical characteristics.

Sandy’s track forecast reasoning remains the same as it has been. The current NNE motion should switch gradually to NNW or NW over the next 24 hours as the mid-level ridge axis to Sandy’s north moves by, changing the steering flow temporarily. After 36-48 hours, Sandy should resume a NNE to NE motion as a large mid-latitude trough comes in from the northwest. Due to Sandy’s likely strong northwest side of the circulation at this time, and the 200mb trough to her west that will be following her northeastward, the storm should easily phase with the longwave trough and be drawn back to the north and northwest into New England by Day 5 and 6. The track forecast has not changed much since yesterday, but has acquired greater confidence in the long range. The GFS, which has been flip-flopping terribly, seems to now be finally settling on a landfall solution instead of a northeast track out to sea, a correction that was expected this model would have to make given its mishandling of the baroclinic dynamics. The ECMWF remains the most consistent of all the models, and the forecast track lies fairly close to the ECMWF ensemble mean.

Sandy has the potential to be a very historic storm for New England, and impacts are expected to be severe over a widespread area. Interests in this region should begin preparations for an event that could surpass Hurricane Irene from last year in many areas.

Sandy has now strengthened rapidly into a hurricane, acquiring heavy wrapped banding that has allowed an eyewall to form, causing a tightening of the wind field. Sandy will make landfall on the eastern end of Jamaica later today, and then quickly move over eastern Cuba. This should slow down Sandy’s strengthening, but should only be a speedbump, and Sandy will likely continue strengthening over the Bahamas. The environment is fairly conducive for strengthening, with a strong divergent flow aloft that will be enhanced by a digging upper trough over the eastern Gulf of Mexico over the next 3 days, and a healthy pressure gradient to the north of the storm that is aiding the rising motion of the air in the Caribbean right now. Dry air will eventually wrap into Sandy from the west and south, but if she continues to deepen, she should be able to maintain some sort of a central core while in the Bahamas. Her NW quadrant is likely to become the strongest part of the storm, and thus the northern Bahamas will likely receive the worst conditions, and the eastern part of Florida will likely see tropical storm conditions.

The long-term track of Sandy remains a bit uncertain, as her potential interactions with New England are still about 6 days away, but my forecast track continues to call for a NW turn into the mid-latitude trough that will be digging strongly into the northeast U.S. during that time. The GFS through the 06z run, which was used when I recorded the video above, still let Sandy escape out to sea, along with the CMC model. However, the 12z GFS has now come around to the idea that Sandy should phase with the trough to the west and curve back into New England. With this new shift from the GFS, the confidence in the forecast track below is reinforced even more. Sandy will be deepening constantly as she phases with the mid-latitude trough, but her wind field will be expanding as she loses tropical characteristics, and thus her maximum wind speed may remain roughly constant as she moves northward. It is hard to say exactly how strong the winds of this hybrid system will be once it nears New England. It will depend on how strong Sandy gets and what her hybrid structure is like. Regardless, this would be one of the strongest autumn storms to ever hit the northeast U.S., and the impacts would be severe. The forecast track lies east of the ECMWF operational and more in line with the ECMWF ensemble mean, given that the ECMWF is likely overdeepening the system and moving it a bit too far west. With 6 days or so to go before New England potentially the wrath of Sandy, there is still plenty of opportunity for the forecast to change, but if the 12z GFS is the start of the models agreeing on this general track forecast, then New England should expect that this will indeed go down as the great Halloween storm of 2012.

Tropical Storm Sandy has formed in the western Caribbean, and continues to gradually deepen. Thunderstorm coverage and density has increased during the last couple of days, and a well-defined center has now formed. The western side of the storm remains void of thunderstorms for now, but that will change as the storm moves northward. Sandy’s present movement is nearly stationary, but she will begin to move northward today in response to a tightening pressure gradient over the Bahamas. This will bring Sandy over Jamaica in 24-36 hours. Steady intensification is expected during this time, and Sandy is expected to be a strong tropical storm and possibly even a minimal hurricane upon landfall in Jamaica. Sandy will then pass over eastern Cuba and into the Bahamas, where the upper-level environment will consist of some southerly wind shear, but also strong divergence aloft east of a negatively-tilted upper trough axis over the Gulf of Mexico, which should allow Sandy to wrap moisture around her center and continue to deepen into a Cat 1 hurricane, possibly a Cat 2 if she tightens her core quickly.

A late season hurricane should be fairly routine for this region to deal with, but the forecast becomes possibly more dangerous in the long term, as Sandy may get captured by a mid-latitude trough and bomb out into a strong hybrid nor’easter in 6-8 days. The model solutions provided by the ECMWF and CMC depict what would be one of the strongest Autumn storms the east coast has ever seen. The video discusses this aspect of the forecast in detail. Right now the GFS is the last model that allows Sandy to escape the trough and move northeastward out to sea. Due to the developmental structure that Sandy is likely to have in the Bahamas which is favorable for phasing, the interaction with the upper trough to her west, and the blocking over maritime Canada, a more westward path than the GFS seems more likely to me at this time. The forecast track is a blend of the ECMWF and UKMET ensembles. The forecast cone indicates the large amount of uncertainty in the 4-6 day forecast, as it is not out of the question that Sandy still tries to escape to the east. Much will depend on the exact timing of the incoming mid-latitude trough, something the models have been varying greatly on, and the convective structure of Sandy in the Bahamas, which can influence her track tendencies. Jamaica, Cuba, the Cayman Islands, Hispaniola, and the Bahamas should all expect direct impacts from Sandy. Florida will likely get some blustery weather, and New England should keep a close eye on this situation, for a storm of the magnitude that is possible in this kind of a situation would be unprecedented, and the potential impacts could be severe.